Sand casting is a widely used method in today's mechanized society. In a typical sand casting process, a pattern is made which conforms to the external shape of the casting. A cavity in sand using the pattern, the cavity having the desired contours and dimensions of the casting. The mold is typically made in an open frame or flask, and both the flask and the pattern are parted to facilitate removal of the pattern from the sand. When the pattern is removed, a cavity is formed in the casting. The mold is closed, and molten metal poured into the mold to fill the cavity. When the metal solidifies, the casting is shaken out and removed, and the sand is reprocessed.
Greensand (or green sand) casting is one of the most popular and widely used methods for producing sand castings. The term “green” refers to the presence of moisture in the molding sand, and demonstrates that the mold is not dried or baked. The mold material comprises sand in quartz form, clay, and water. The water develops the bonding characteristics of the clay, which binds the sand grains together. Through the application of pressure, mold material is compacted around a pattern to produce a mold with sufficient rigidity to enable molten metal to be poured into it to produce a casting.
Defects can occur in the mold formation process from a variety of sources: moisture level in the green sand, organic impurities, poor ram or squeeze, hard molds, uneven mold hardness, pattern problems, insufficient new sand, and contaminated sand, among others. Greensand characteristics are normally measured from the muller, and are limited to information on the greensand at that time. From the muller, the greensand travels along conveyors, down hoppers, and through nozzles into the mold chambers. Any of these processes can affect the resulting mold properties. In addition, after the greensand mold is made, it is moved to the pour station, and during this movement, the mold may develop cracks or may even collapse. Problems with a mold usually cannot be detected by visual inspection. Unfortunately, undiscovered defects in the sand mold lead to a defective casting, resulting in scrapping of the casting. As the cost of energy rises and competition for higher quality casting of greensand molds increases, the ability to reduce scrap rates and identify process irregularities and problems which affect quality of the casting become increasingly important.